High-power ultrashort pulse lasers have wide range of applications in scientific research, industrial material processing, and life science. Traditionally, high-power ultrashort pulses are generated using the mode-locking technique of solid-state lasers, which are bulky, expensive, and difficult to operate. Passively mode-locked fiber lasers are an attractive alternative to solid-state lasers for generating femtosecond optical pulses. Fiber lasers are compact in size, cost effective and easy to operate. Several methods can be used to achieve passive mode locking in fiber lasers, for example the semiconductor saturable absorber mirror (SESAM) method, the nonlinear loop mirror (NLP) method and the nonlinear polarization rotation (NPR) method. However, due to its simplicity, stability and ultrafast recovery time, the NPR method has attracted great attention and is widely used. Using the NPR technique to generate femtosecond optical pulses in the erbium-doped fiber lasers is now a routine work.
However, a fundamental drawback of the passively mode-locked fiber lasers is that the generated optical pulses have only low pulse energy and peak power in the respective range of several pico-joules and several hundred watts. It was generally believed that the soliton operation of the fiber lasers limited the maximum achievable pulse energy and peak power. The NPR mode-locking action becomes saturated when the nonlinear phase shift of a mode-locked pulse accumulated within one cavity round-trip exceeds a certain value. Under soliton operation of the lasers this value can be easily reached.
To boost energy of the mode-locked pulses, a number of techniques were implemented. One is the stretched-pulse technique as disclosed in publication “Stretched-Pulse Additive Pulse Mode-Locking in Fiber Ring Lasers: Theory and Experiment”, H. A. Haus et al., IEEE Journal of Quantum Electronics. Vol 31, No. 3, March 1995. The idea of the stretched-pulse technique is to use fibers of large positive and negative group velocity dispersion (GVD) to construct the fiber laser cavity. As an optical pulse circulating in such a laser cavity is temporally stretched and compressed in one transit, the average peak power of the mode-locked pulse becomes lower than that of a transform-limited pulse of the same spectral bandwidth. Therefore, the effective nonlinear phase shift accumulated in one cavity round-trip is smaller, and pulse with larger energy can be obtained. The output pulses of stretched-pulse fiber lasers generally have picosecond duration and are linearly chirped. To obtain sub-100 fs pulses the laser output needs to be further compressed through an external pulse compression system, which makes the lasers inconvenient to use.
Another technique of generating high power ultrashort pulses from the fiber based systems is the so-called Master Oscillator and Power Amplification (MOPA) method as disclosed in publication “High-power Ultrafast Fiber Laser Systems”, Jens Limpert et al., Journal of Selected Topics in Quantum Electronics, Vol. 12, No. 2, March/April 2006, where light pulse from a stable low power fiber laser is first power amplified by a fiber amplification system, and then linearly compressed. Current high power ultrashort fiber laser systems are made based on this technique. In this fiber system apart from the laser oscillator, it also has an external cavity pulse amplification part and a pulse compression part, which may complicate the system.
Therefore, an objective of the present invention is to provide an alternative method to generate high peak power ultrafast near transform-limited pulses directly from the passively mode locked fiber lasers thereby advantageously avoids or reduces some of the above-mentioned drawbacks of prior art devices. It is also an objective of the present invention to provide an alternative laser arrangement which can be used with the method for better performance.